use dates rather than datetimes when comparing with bst_date

bst_date is a date field. Comparing dates with datetimes in postgres
gets confusing and dangerous. See this example, where a date evaluates
as older than midnight that same day.

```
notification_api=# select '2019-04-01' >= '2019-04-01 00:00';
 ?column?
----------
 f
(1 row)
```

By only using dates everywhere, we reduce the chance of these bugs
happening

app/dao/date_util.py

@@ -1,4 +1,4 @@
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, date, time
 
 from notifications_utils.timezones import convert_bst_to_utc
 import pytz
@@ -64,6 +64,9 @@ def get_current_financial_year_start_year():
 
 
 def get_financial_year_for_datetime(start_date):
+    if type(start_date) == date:
+        start_date = datetime.combine(start_date, time.min)
+
     year = int(start_date.strftime('%Y'))
     if start_date < get_april_fools(year):
         return year - 1